The output of present day high pressure drum type steam generators for utility application is limited by the size and weight of the steam-water drum which can be shipped from the manufacturing facilities to the erection site. In very large steam generators, the drum length required to accommodate the necessary steam-water separating equipment may exceed 120 feet and its weight could be more than 500 tons. Needless to say, a drum of this size is very difficult to transport to the erection site and to install on a steam generator.
An arrangement of two steam-water drums connected in series has been used by the prior art when faced with crude and inefficient steam-water separating equipment. This series arrangement requires that the steam-water mixture flowing from the generating tubes be first introduced into one of the two steam-water drums for primary separation of steam and water. The separated moisture laden steam is then conveyed to the other steam-water drum for secondary separation and the drying of the separated steam. This series arrangement of steam-water drums wherein primary and secondary separation of steam and water takes place in separate drums is too costly since it permits only a slight reduction in the size of each drum relative to that of a drum capable of accommodating both steam-water separating stages.
An arrangement of two steam-water drums connected in parallel has been used by the prior art as disclosed in Bell, U.S. Pat. No. 1,036,517; Ulrich, U.S. Pat. No. 1,917,617 and Stevens, U.S. Pat. No. 3,662,716. However, this parallel arrangement does not provide for uniform distribution of steam and water to and from each drum. For example, the riser tubes delivering steam to the drums are routed for layout convenience rather than uniform distribution between drums thereby leading to non-uniform steam output from each drum. In cases where each drum is provided with its own downcomer pipes, the non-uniform steam output leads to different enthalpies in the water flowing through the downcomer pipes from each drum to the various furnace generating circuits and causes non-uniform circulation rates through these circuits thereby resulting in unequal steam delivery to the two drums. This prior art arrangement encounters large water level difference in each drum that affects the separation of steam and water and leads to undesirable water level fluctuations and associated difficulties of water level control.